Recent versions of youtube-dl handles recombination if ffmpeg is in PATH

ytdl

#!/bin/bash
#Youtube-DL DASH Video and Audio merging script
#Written by QuidsUp
#Edited by Christoph Korn
#Edited by William Leung
#Edited by Joel Thieffry

URL=$1
if [ -z $URL ]; then
  echo "Usage: ytdl.sh url"
  exit
fi

#Find what quality of videos are available
youtube-dl -F $URL
echo
echo -n "Quality for Video (default 137): "
read Qual1
echo -n "Quality for Audio (default 141): "
read Qual2

#Set values if user has just pressed Return without typing anything
if [ -z $Qual1 ]; then
  Qual1="137"
fi
if [ -z $Qual2 ]; then
  Qual2="141"
fi

#Set filenames from output of youtube-dl
File1=$(youtube-dl --get-filename -f $Qual1 $URL)
echo $File1

#Download Video file with First Quality Setting
#       + Audio file with Second Quality Setting
# then recombine them with ffmpeg
youtube-dl -f $Qual1+$Qual2 $URL
if [[ ! -f $File1 ]]; then
  echo
  echo "Error video file not downloaded"
  exit
fi

echo "Video file downloaded successfully"